In recent years, in the field of an image forming technology based on electrophotography, increased demand has arisen for full-color image formation capable of providing images with higher image quality, and thus, developers have been designed so as to provide high-quality images. In order to cope with the demand for the improved image quality, particularly in full-color images, there is an increasing tendency toward the production of toners having smaller particle diameters, and studies have been made on faithful reproduction of latent images. Regarding the reduction in particle diameter, a process for producing a toner by a polymerization process has been proposed as a method that can regulate the toner so as to have desired shape and surface structure (see, for example, PTLs 1 and 2). In the toner produced by the polymerization process, in addition to the control of the diameter of toner particles, the shape of toner particles can also be controlled. A combination of this technique with a particle size reduction can improve the reproducibility of dots and thin lines, and can reduce pile height (image layer thickness), whereby an improvement in image quality can be expected. The polymerized toner generally contains a binder resin, a colorant, a charge-controlling agent and other additives.
Conventionally, various charge-controlling agents have been proposed to impart to toners excellent charging property, stability over time and environmental stability. In this case, since a colored material cannot be used in a charge-controlling agent for use in full-color toners, there must be used colorless, white or light-colored charge-controlling agents which do not affect the hue of the toner.
Examples of such charge-controlling agents proposed include metal complex salts of salicylic acid derivatives (see PTLs 3 to 6), metal salts of aromatic dicarboxylic acids (see PTL 7), metal complex salts of anthranilic acid derivatives (see PTL 8) and organic boron compounds (see PTLs 9 and 10).
However, these charge-controlling agents have disadvantages that they contain chromium which may be unstable to the environment, and have insufficient durability, charge-imparting effects and environmental stability. Thus, they do not have sufficient performance to be used successfully as a charge-controlling agent. Also, as a metal-free charge-controlling agent, condensates of phenol derivatives have been proposed (see PTL 11). These condensates may satisfactorily meet the requirements of a charge-controlling agent.
As described above, in the polymerized toner, the charge-controlling agent derived from the toner material may be decomposed, or difficult to disperse in the toner. In many cases, the charge-controlling agent cannot sufficiently exhibit its functions, which is problematic. Therefore, there have been no toners excellent in chargeability, durability and environmental stability by using a charge controlling agent applicable to a polymerized toner, having smaller particle diameter and forming high-quality images. In addition, the relevant techniques to the formation of such toners have not yet been provided. Therefore, keen demand has arisen for such toners and techniques.